Electrochemical sensor

ABSTRACT

An organic contaminant molecule sensor is described for use in a low oxygen concentration monitored environment. The sensor comprises an electrochemical cell, which is formed from a measurement electrode coated with (or formed from) a catalyst having the ability to catalyse the dissociative adsorption of the organic contaminant molecule, the electrode being positioned for exposure to the monitored environment, a reference electrode coated with (or comprised from) a catalyst selected for its ability to catalyse the dissociation of oxygen to oxygen anions, the reference electrode being positioned within a reference environment, and a solid state oxygen anion conductor disposed between and bridging the measurement and reference electrodes, wherein oxygen anion conduction occurs at or above a critical temperature, T c . Sealing means are provided for separating the reference environment from the monitored environment. Means are also provided for controlling and monitoring the temperature of the cell, and for controlling the electrical current (I p ) flowing between the reference and measurement electrodes. At temperatures (T ads ) below T c , organic contaminant molecules are adsorbed onto and dissociated at the surface of the measurement electrode leading to the build up of carbonaceous deposits at the surface thereof. At temperatures (T tit ) above T c , an electrical current (I p ) is passed between the reference and measurement electrode thereby to control the number of oxygen anions passing from the reference electrode to the measurement electrode to oxidise the carbonaceous deposits formed at the surface thereof and the formation of carbon dioxide.

This invention relates to a sensor for the detection of organiccontaminants in low oxygen concentration process environments, such asthose used in the semiconductor manufacturing industry, the use of suchsensors and a novel method for the detection of organic contaminants insuch process environments. The term “low oxygen concentration processenvironment” is to be understood to mean a process environment in whichthe partial pressure of oxygen is of the order of 10⁻⁶ mbar to 10⁻³ mbar(parts per billion to parts per million).

In, for example, the semiconductor manufacturing industry, it isimportant to control the atmosphere (the process environment) in whichwafers are manufactured. The wafers are desirably manufactured in acontrolled environment, as undesirable or varying levels of organiccontaminants can result in device and/or equipment failure.

Levels of contaminating organic material in the parts per trillion (ppt)to parts per billion (ppb) range, which corresponds to a partialpressure of 10⁻⁹ mbar to 10⁻⁶mbar, do not, in general, result in deviceor equipment failure. However, if the levels of organic contaminantsbecome much higher than this, failures may result. In order to controlthe process environment, it is necessary to monitor the levels oforganic contaminants present. In particular, some processes aresensitive to contaminant material in the low ppb range, and so for theseprocesses it is desirable to monitor the level of contaminant materialsin the ppt range. However, such monitoring processes are costly and itis difficult to determine an accurate value for the total organiccompounds (TOC) present at such low contaminant levels. In addition,many fabrication processes are tolerant of light saturated hydrocarbons,such as methane (CH₄) and ethane (C₂H₆), which have particularly lowreaction probabilities with most surfaces and therefore do not take partin the various contamination inducing reactions.

In vacuum based process environments, TOC levels are often determinedusing mass spectrometry, as a mass spectrometer is capable of measuringcontamination levels of the order of ppt. However the interpretation ofsuch measurements is often complicated by effects such as mass spectraloverlap, molecular fragmentation and background effects, for example.

Although mass spectrometers can be used in process environmentsoperating at ambient pressure or above, additional vacuum and samplehandling systems are required, which make such instruments veryexpensive. Under such conditions, it is preferred to use gaschromatographic techniques to monitor the TOC levels present in theprocess environment. However, in order to monitor contaminants in theppt range it is necessary to fit the gas chromatogram with a gasconcentrator.

It should be noted that although mass spectrometry and gaschromatography are able to detect ppt levels of TOC, their ability todifferentiate the presence of the process-tolerant light hydrocarbonsreferred to above from the more harmful organic compounds is limited,which makes it difficult to determine the total levels of damaginghydrocarbons in the process environment.

In addition, because the use of either mass spectrometric or gaschromatographic techniques for determining the TOC levels present inprocess environments requires specialist equipment, they tend to berather expensive and are typically only used as Point of Entry (POE)monitors for the whole facility rather than the more useful Point of Use(POU) monitors.

Hydrocarbons, including light hydrocarbons such as methane (CH₄) andethane (C₂H₆), have been routinely monitored using common tin oxide(SnO₂) based sensor devices. These sensors typically operate underatmospheric pressure to detect target gases in the range from tens ofppb (parts per billion) to several thousand ppm (parts per million).This type of sensor works effectively within these ranges by providing alinear output signal that is directly proportional to the quantity oftarget gas within the monitored environment. Although these sensors aresuitable for monitoring contaminant levels within ambient environments,they do not lend themselves for applications with sub-atmosphericprocessing environments such as those encountered within semiconductorprocessing environments. Under such vacuum conditions the SnO₂-type ofsensor will suffer from reduction of the active oxide content leading tosignal drift and non-response after a period of time.

Chemical sensors comprising solid state electrolytes such as oxygenanion conductors, or silver or hydrogen cation conductors, have beenused to monitor levels of oxygen, carbon dioxide, and hydrogen/carbonmonoxide gas present in a process environment and are described inUnited Kingdom patent application number 0308939.8 and GB 2,348,006A, GB2,119,933A respectively. Such sensors are generally formed from anelectrochemical cell comprising a measurement electrode, a referenceelectrode and a solid state electrolyte of a suitable ionic conductordisposed between and bridging said electrodes.

For example, the gas monitor of GB 2,348.006A comprises a detectionelectrode containing a silver salt having an anion, which corresponds tothe gas to be detected, a silver ion conducting solid state electrolyteand a reference silver electrode. The gas monitor can be used to detectgases such as carbon dioxide, sulphur dioxide, sulphur trioxide,nitrogen oxides and halogens through the suitable choice of theappropriate anion.

For the oxygen sensors of United Kingdom patent application number0308939.8, the solid state electrolyte conducts oxygen anions and thereference electrode is generally coated or formed from a catalyst thatis able to catalyse the dissociative adsorption of oxygen and ispositioned within a reference environment, in which the concentration ofoxygen adjacent the reference electrode remains constant.

Solid state oxygen anion conductors (solid state electrolytes) aregenerally formed from doped metal oxides such as gadolinium doped ceriaor yttria stabilised zirconia (YSZ). At temperatures below the criticaltemperature for each electrolyte (T_(c)) the electrolyte material isnon-conducting. At temperatures above T_(c) the electrolyte becomesprogressively more conductive.

Oxygen levels as determined by such sensors in any monitored environmentis determined by the electrochemical potentials generated by thereduction of oxygen gas at both the measurement and referenceelectrodes. The steps associated with the overall reduction reactions ateach electrode are set out below, the half-cell reaction at eachelectrode being defined by equations 1 and 2 below.O_(2(gas))⇄2O_((ads))  (Equation 1)O_((ads))+2e⁻⇄O²⁻  (Equation 2)

The electrochemical potential generated at each electrode is determinedby the Nernst equation: $\begin{matrix}{E = {E^{\Theta} + {\frac{RT}{2F}\ln\frac{a\left( O_{ads} \right)}{a\left( O^{2 -} \right)}}}} & {{Equation}\quad 3}\end{matrix}$where

-   E is the electrochemical half-cell potential at the reference or    measurement electrode respectively;-   E^(Θ) is the standard electrochemical half cell potential of the    cell at unit O_((ads)) activity-   R is the gas constant-   T is the temperature of the cell-   F is Faraday's constant-   a(O_(ads)) and a(O²⁻) are the activities of the adsorbed oxygen at    the electrode surface and reduced oxygen anion in the solid state    ionic conductor respectively.

The activity of adsorbed oxygen at the electrode surface is directlyproportional to the partial pressure of oxygen gas, P_(O2), in theenvironment adjacent the electrode as defined by equation 4 below:a(O_(ads))=KP^(1/2) _(O) ₂   Equation 4

Since a(O²⁻) is unity, by definition, and the activity of the adsorbedoxygen at the electrode surface is proportional to the partial pressureof the oxygen in the environment adjacent the electrode surface(equation 4), the half cell potential can be written in terms of thepartial pressure of oxygen in the particular environment adjacent themeasurement or reference electrode respectively $\begin{matrix}{E = {E^{\Theta} + {\frac{RT}{4F}\ln\quad p_{O_{2}}}}} & {{Equation}\quad 5}\end{matrix}$

The potential difference V generated across the cell is defined in termsof the difference in the half-cell potentials between the reference andmeasurement electrodes in accordance with equation 6. $\begin{matrix}{V = {{E_{(R)} - E_{(M)}} = {\frac{RT}{4F}{{Ln}\left( \frac{P_{O\quad 2{(R)}}}{P_{O\quad 2{(M)}}} \right)}}}} & {{Equation}\quad 6}\end{matrix}$where

-   V is the potential difference across the cell-   E_((R)) and E_((M)) are the electrochemical potentials at the    reference and measurement electrodes respectively;-   R, T and F are as defined above; and-   P_(O2(R)) and P_(O2(M)) are the partial pressures of oxygen at the    reference and measurement electrodes respectively.

Note that if both the reference and measurement electrodes are exposedto the same oxygen partial pressure e.g. atmospheric levels of oxygen,the potential difference across the cell is zero. In processenvironments such as the oxygen deficient environments encountered inthe manufacture of semiconductor products the partial pressure of oxygenadjacent the measurement electrode is considerably less than thatadjacent the reference electrode. Since the electrochemical potential ateach electrode is governed by the Nernst equation, as the partialpressure of oxygen at the measurement electrode decreases, theelectrochemical potential at the measurement electrode changes, whichresults in the formation of a potential difference across the cell attemperatures above the critical temperature. The potential differenceacross the cell is determined by the ratio of the partial pressure ofoxygen at the reference and measurement electrodes in accordance withequation 6 above. The oxygen sensor can therefore provide a user with anindication of the total amount of oxygen present in a monitoredenvironment simply from determining the potential difference across thecell.

However, there is a need for a similar simple, low cost,semi-quantitative sensor, which has a low sensitivity to unreactiveorganic compounds but can be used at the point of use to analyse theprocess environment. In at least its preferred embodiment, the presentinvention seeks to address that need.

A first aspect of the present invention provides an organic contaminantmolecule sensor for use in a low oxygen concentration monitoredenvironment, the sensor comprising an electrochemical cell comprising asolid state oxygen anion conductor in which oxygen anion conductionoccurs at or above a critical temperature T_(c), a measurement electrodeformed on a first surface of the conductor for exposure to the monitoredenvironment, the measurement electrode comprising material forcatalysing the dissociative adsorption of the organic contaminantmolecule, and a reference electrode formed on a second surface of theconductor for exposure to a reference environment, the referenceelectrode comprising material for catalysing the dissociation of oxygento oxygen anions; means for controlling and monitoring the temperatureof the cell; and means for controlling the electrical current flowingbetween the reference and measurement electrodes, whereby attemperatures below T_(c), organic contaminant molecules are adsorbedonto and dissociated at the surface of the measurement electrode leadingto the build up of carbonaceous deposits at the surface thereof, and attemperatures above T_(c), an electrical current is passed between thereference and measurement electrode thereby to control the number ofoxygen anions passing from the reference electrode to the measurementelectrode to oxidise the carbonaceous deposits formed at the surfacethereof and the formation of carbon dioxide.

In the absence of organic contaminants, at temperature T_(tit) andconstant electrical current I_(p), the potential difference across thecell, V_(o), is constant and is determined by the equilibrium betweenthe flux of oxygen anions (O²⁻) arriving at the electrode surface andthe rate of desorption of oxygen gas (O_(2(g))) from the electrodesurface according to equations 1 and 2 above.

However, when carbonaceous deposits are present on the electrode surfacethey are oxidised (combusted) to carbon dioxide by the flux of oxygenanions arriving at the electrode surface. This has the effect ofreducing the equilibrium concentration of oxygen anions (O²⁻) at thesurface, which means that, in accordance with equation 3 above, thepotential across the cell, V_(tit), is increased relative to V_(o). Whenthe current I_(p) is applied to the cell, oxygen anions are forced toflow from the reference electrode to the measurement electrode wherethey react with the carbonaceous deposits formed at the surface thereofduring the adsorption phase, which results in the formation of carbondioxide. As the carbonaceous deposits are transformed into carbondioxide their concentration progressively decreases to zero at thesurface of the measurement electrode and the concentration of oxygen atthe surface of the measurement electrode will increase to the constantequilibrium value determined by the flux of oxygen anions to theelectrode. The potential difference across the cell then returns to theconstant value, V_(o), and provides an indication that all thecarbonaceous deposits on the surface of the electrodes have beenremoved.

The total amount of carbonaceous deposit formed at the surface of themeasurement electrode can be determined by measuring the total amount ofoxygen transported to the measurement electrode by the application ofcurrent I_(p) (which is required to oxidise all of the carbonaceousdeposits) over the time t_(p) taken for the potential difference acrossthe cell to return from V_(tit) to V_(o). Since the transport of eachoxygen anion to the surface of the measurement electrode requires thepassage of two units of charge, the total quantity of oxygen atomstransported to the surface of the electrode is determined by the term:$\begin{matrix}\frac{I_{p}t_{p}}{2} & {{Equation}\quad 7}\end{matrix}$

Since each atom of carbon deposited at the surface of the electroderequires two oxygen atoms for complete combustion, the total amount ofcarbon atoms oxidised during the titration phase and hence deposited atthe electrode during the adsorption phase is: $\begin{matrix}\frac{I_{p}t_{p}}{4} & {{Equation}\quad 8}\end{matrix}$

It will be appreciated that by controlling the time over whichadsorption of the organic contaminants can occur on the surface of themeasurement electrode, the value of the current I_(p) flowing betweenthe reference and measurement electrodes at temperature T_(tit) and thetime t_(p) taken for the potential difference across the cell to dropfrom V_(tit) to V₀, it is possible to titrametrically monitor the levelsof organic contaminants in the process environment in the parts pertrillion (ppt) range or less. The sensor therefore provides a low costalternative to the use of mass spectrometry and gas chromatography inthe determination of low levels of organic impurities in processenvironments.

The reference environment may, for example, be a gaseous source ofoxygen at constant pressure (such as atmospheric air) or a solid-statesource of oxygen, typically a metal I metal oxide couple such as Cu/Cu₂Oand Pd/PdO or a metal oxide/metal oxide couple such as Cu₂O/CuO.

The sensor is also easy to use and can be used at the point of use aswell as the point of entry to provide accurate information about theprocess environments at all stages of the semiconductor fabricationprocess.

The total level of contaminants measured by the sensor can provide asemi-quantitative indication of the level of harmful organiccontaminants present in the process environment. The non-contaminatinglight organic molecules present in the process environment do not stickto the surface of the measurement electrode and are not thereforemeasured. It is only the harmful organic contaminants, which have a highreaction probability with the electrode surface (and therefore withother surfaces encountered in the fabrication process) that undergodissociation and are therefore subsequently oxidised at the measurementelectrode surface that are detected and therefore monitored by themeasurement electrode.

Careful choice of the material applied to the measurement electrode orthe material from which it is formed will cause some of the harmfulorganic contaminants to adsorb onto the surface of the measurementelectrode in preference to others. Preferably the measurement electrodeis formed from material whose uptake of organic material proceeds with asticking probability of or about unity. In addition, the organicmaterial is preferably efficiently adsorbed and cracked by the electrodematerial. Furthermore, the measurement electrode is suitably able tocatalyse the dehydrogenation and cracking of organic contaminants.Suitable electrode materials include metals selected from the groupcomprising rhenium, osmium, iridium, ruthenium, rhodium, platinum andpalladium and alloys thereof. Alloys of the aforementioned materialswith silver, gold and copper may also be used.

The sensor is easily and readily manufactured using techniques known toa person skilled in the art. Sensing, reference and optionally counterelectrodes can be applied to a thimble of an oxygen anion conductorsolid state electrolyte such as ytttria stabilised zirconia either inthe form of an ink or a paint or using techniques such as sputtering.The sensing electrode is isolated from the reference and optionalcounter electrode via the formation of a gas tight seal. The sensor issuitably supplied with heater means to control the temperature of theelectrolyte and means to monitor the voltage between the sensingelectrode and the reference and counter electrodes respectively.

The reference electrode is suitably formed from a material that is ableto catalyse the dissociation of oxygen in the reference environment, forexample platinum. The reference environment can be derived from agaseous or solid-state source of oxygen. Typically atmospheric air isused as a gaseous reference source of oxygen although other gascompositions can be used. Solid-state sources of oxygen typicallycomprise of a metal/metal oxide couple such as Cu/Cu₂O and Pd/PdO or ametal oxide/metal oxide couple such as Cu₂O/CuO. The particularsolid-state reference materials chosen will depend on the operatingenvironment of the sensor and in particular the titration temperatureT_(tit). The solid state electrolyte comprising an oxygen anionconductor is suitably formed from a material that exhibits oxygen anionconduction at temperatures above 300° C. Suitable oxygen anionconductors include gadolinium doped ceria and yttria stabilisedzirconia. Preferred materials for use as the solid state oxygen anionconductor include 8% molar yttria stabilised zirconia (YSZ), which iscommercially available.

A radiative heater may be used to control the temperature of the cell.Such heaters include heating filaments, wound around the solid stateelectrolyte. An electric light bulb can also be used. A thermocouple maybe used to monitor the temperature of the cell.

Currents of between 100 nA and 100 μA may be used for driving oxygenanions between the reference and measurement electrodes. Currentsoutside this range can be used, depending upon the circumstances. Themagnitude of the current used to drive the oxygen anions between thereference and measurement electrodes depends upon the surface area ofthe electrode and the amount of cracked hydrocarbon deposited at thesurface thereof. Larger currents will generally be required forelectrodes having a greater surface area or a large amount of crackedhydrocarbon deposited on the surface thereof. The sensor is preferablyused in conjunction with a device for measuring the potential producedacross the cell.

In use, the sensor continuously cycles between an adsorption mode and anoxygen titration mode:

In the adsorption mode, the sensor is held at a constant temperature,T_(ads), which is below the critical temperature, T_(c), for oxygenanion conduction within the solid state electrolyte. The T_(c) for YSZ,for example, is in the range 300° C. T_(ads) and the sensing electrodematerial are chosen such that the catalytic properties of the sensingelectrode, at T_(ads), cause adsorbed organic material tode-hydrogenate/crack leading to the build up of carbonaceous deposits onthe surface. For a platinum electrode, for example, T_(ads) is in therange 20 to 80° C. In the ideal case complete de-hydrogenation/crackingwill occur leaving a surface layer of adsorbed carbon.

The sensor is held at temperature, T_(ads), for a time, t_(ads), duringwhich adsorption of the organic contaminants occurs. The length of time,t_(ads), is suitably between 10 and 10⁵ seconds and is preferably of theorder of 10²-10³ seconds. Greater sensitivities can be achieved usinglonger adsorption times. It is, however, desirable that saturation ofthe measurement electrode during the adsorption phase is avoided as thiswill change the sticking/reaction probability of the surface, typicallysurface coverages of <0.5 monolayers are desirable. In the event thatsaturation of the electrode occurs this can be overcome by burning thecarbonaceous deposit off of the surface of the measurement electrode andre-adsorbing for a shorter period of time.

In the event that incomplete cracking of the hydrocarbon contaminantoccurs during the adsorption phase, complete cracking can be achieved byraising the temperature of the sensor to a temperature intermediate thatof the adsorption temperature, T_(ads), and the titration temperature,T_(tit), and optionally applying a potential V_(l) across the sensor,under which conditions complete cracking of the adsorbed hydrocarbonoccurs. In some applications, V_(l) may be zero. The sensor is held atthe intermediate temperature for a period of time sufficient to allowcomplete conversion of any uncracked hydrocarbon contaminant to acarbonaceous deposit on the surface of the measurement electrode.

In the oxygen titration mode, the sensor temperature is rapidly raisedfrom either the adsorption temperature, T_(ads), or the intermediatetemperature to a fixed absolute temperature, T_(tit), which is aboveT_(c). During this temperature ramp de-hydrogenated organic materialwill remain on the surface. Once at T_(tit), a small current I_(p) isforced to pass through the electrochemical cell, pumping oxygen to thesensing electrode surface, as per equation 9. Currents of the order of100 nano-amps to 100 micro-amps are suitably used. The oxygenprogressively combusts the carbonaceous residue on the sensing electrodein accordance with equation 10.2O²⁻→2O(ads)+4e⁻  Equation 9C(ads)+2O(ads)+2O(ads)→CO₂  Equation 10

The potential of the sensing electrode, relative to the referenceelectrode, will tend to the equilibrium thermodynamic value predicted bythe Nernst equation 3, as the titration reaction proceeds. When theequilibrium thermodynamic voltage V_(o) is reached the oxygen titrationreaction is complete. The total amount of charge that has flowed throughthe cell during the titration reaction is directly proportional to theamount of oxygen reacted. For the ideal case the amount of carbonaccumulated during the adsorption phase will be ¼ of the total charge,as per equations 9 and 10. The amount of carbon can be determined fromthe time taken in the adsorption phase and the surface area of thesensing electrode. Wet potentiometric titration procedures and cyclicvoltammetry are frequently used to monitor the levels of knowncontaminant species in solutions. All of these processes take place inthe liquid phase and use reversible electrode processes, mostly usingwater as the major constituent of the solvent and use electrons todirectly effect oxidation/reduction. Gas phase electrochemistry isconfined to the areas of electrochemical sensors, both potentiometricand amperometric, and solid oxide fuel cells neither of which use anykind of titration reaction.

It will therefore be appreciated that, in contrast to the wettitrametric procedures of the prior art, the sensor facilitates thetitrametric determination of trace organic contaminants in the gaseousphase using a solid state electrolyte.

Although the sensor can be used with just two electrodes (the referenceand measurement electrode) only, it is preferred to use an electrodearrangement comprising a counter electrode in addition to themeasurement and reference electrodes as described above. The counterelectrode is positioned adjacent to the reference electrode and incontact with the same reference environment as the reference electrode.In this preferred embodiment, the current I_(p) flows between thecounter electrode and the measurement electrode. The reference electrodetherefore provides a constant reference environment from which theelectrochemical potentials of both the measurement and counterelectrodes and therefore the potential difference across the cell can bedetermined. The counter electrode is preferably formed from a material,such as platinum, which catalyses the dissociation of oxygen.

The dimensions of the top and bottom surfaces of the sensor aretypically of the order of a few square centimetres or less. Theelectrodes formed or deposited on each of the surfaces are thereforedimensioned accordingly. The sensing and counter electrodes are eachtypically of the order of 1cm². The reference electrode is usually of alesser dimension. The electrodes are typically from about 0.1 to about50 μm in thickness.

It will be appreciated that the sensor can be used to monitor the levelsof trace organic contaminants in process environments, and so a secondaspect of the invention provides the use of a sensor as aforementionedto monitor levels of trace organic contaminants in process environments.

It will further be appreciated that the sensor of the first aspect ofthe invention can be used in a method for monitoring the level of traceorganic contaminants in process environments. A third aspect of theinvention thus provides a method of monitoring the levels of traceorganic contaminants in a monitored process environment, the methodcomprising the steps of providing an electrochemical sensor comprising asolid state oxygen anion conductor in which oxygen anion conductionoccurs at or above a critical temperature T_(c), a measurement electrodeformed on a first surface of the conductor for exposure to the monitoredenvironment, the measurement electrode comprising material forcatalysing the dissociative adsorption of the organic contaminantmolecule, and a reference electrode formed on a second surface of theconductor for exposure to a reference environment, the referenceelectrode comprising material for catalysing the dissociation of oxygento oxygen anions; exposing the measurement electrode at a sensortemperature T_(ads) to the monitored environment for a time t_(ads) tocause one or more organic contaminant species to be adsorbed onto anddehydrogenate at the surface of the measurement electrode therebyleading to the build up of a carbonaceous deposit at the surfacethereof; raising the temperature of the sensor to a value T_(tit) abovethe critical temperature T_(c) of the solid state oxygen anion conductorand passing a current I_(p) between the reference electrode and themeasurement electrode for a time t_(p) taken for the potentialdifference across the sensor to reach a constant value determined by theequilibrium between the flux of oxygen anions arriving at the electrodesurface and the rate of desorption of oxygen gas from the electrodesurface; and determining from the total charge (I_(p)t_(p)) passedthrough the sensor at temperature T_(tit) the amount of carbonaceousdeposit present at the surface of the measurement electrode andtherefore the concentration of organic contaminant species present inthe process environment.

Preferred features of the present invention will now be described, byway of example only, with reference to the accompanying drawings, inwhich:

FIG. 1 illustrates a first embodiment of an electrochemical sensor;

FIG. 2 illustrates a second embodiment of an electrochemical sensor; and

FIG. 3 illustrates the relationship between the voltage across the celland the total charge passed therethrough during the titration timet_(tit).

The electrochemical sensor of FIG. 1 comprises a measurement electrode10 deposited on one side of a solid state electrolyte 12 comprising anyttrium stabilised zirconium oxygen anion conductor tube. Themeasurement electrode may be deposited using a technique such as vacuumsputtering or applying any suitable commercially available “ink” to thesurface. In the event that the measurement electrode 10 is formed on thesurface of the electrolyte 12 using ink, the whole assembly must befired in a suitable atmosphere determined by the nature of the ink. Inthe preferred embodiment, the measurement electrode 10 is formed fromplatinum. Alternatively, the measurement electrode 10 may be formed fromany other material that is able to catalyse the dehydrogenation of ahydrocarbon contaminant such as propylene to carbonaceous material atits surface. In use the measurement electrode 10 is placed in contactwith a monitored environment 14.

A reference electrode 16 is formed on the opposite surface of theelectrolyte 12 to the measurement electrode 10 using similar techniquesto those described above for measurement electrode 10. In the preferredembodiment, the reference electrode 16 is formed from platinum.Alternatively, the reference electrode 16 may be formed from any othermaterial that is able to catalyse the dissociation of oxygen to oxygenanions. In use, the reference electrode 16 is placed in contact with areference environment 18, which, in this embodiment, is a gaseous sourceof oxygen at constant pressure such as atmospheric air. The electrodes10, 16 and the electrolyte 12 together form an electrochemical cell 14.

The sensor is mounted in the environment to be monitored using amounting flange 20, and the measurement electrode 10 is typicallyisolated from the reference electrode 16 through the use of gas tightseals 22. In this way it is possible to separate the monitoredenvironment 14 from the reference electrode 16 and the referenceenvironment 18.

The sensor is provided with a heater and thermocouple assembly 24 forheating the sensor and for providing an indication of the temperature ofthe sensor. The heater and/or thermocouple may be, as illustrated, aself contained cartridge assembly, or may be bonded to the electrolyteprior to the formation of the electrodes; sputtered onto the electrolytesubsequent to the formation of the electrodes or wound round theelectrolyte prior to or subsequent to the isolation of the sensingelectrodes from the reference and counter electrodes. The temperature ofthe sensor is controlled by a suitable control device 26.

A constant current source 28 is also provided to control the currentflowing from the reference electrode 16 to the measurement electrode 10.A voltammeter 30 is also provided to measure the potential differenceacross the cell.

In use the sensor is cycled between an adsorption phase and a titrationphase. In the adsorption phase, the measurement electrode 10 is exposedto an environment to be monitored, including any organic contaminants.The sensor is held at a temperature T_(ads) for a time t_(ads) duringwhich any organic contaminants are adsorbed onto and dehydrogenated atthe surface of the measurement electrode 10, resulting in the formationof carbonaceous deposits thereat. The sensor then enters the titrationphase. The temperature of the sensor is raised to a temperature T_(tit)above the critical temperature T_(c) of the electrolyte at which theelectrolyte becomes conducting. Once at temperature T_(tit) a knowncurrent I_(p) is passed between the reference electrode 16 and themeasurement electrode 10 thereby to force oxygen anions to pass from thereference electrode to the measurement electrode where they are oxidisedand react with the carbonaceous deposits formed at the surface of themeasurement electrode during the adsorption phase to form carbondioxide.

During the titration phase the potential difference across the cell ismonitored, together with the time taken for the potential difference toreach a constant value, V_(o), characterised by the current I_(p). Theestablishment of this constant potential indicates that sufficientoxygen has passed between the reference and measurement electrodes tocause complete oxidation of all of the carbonaceous deposits present atthe surface of the measurement electrode 10 and therefore that the endpoint of the oxidative titration has been reached.

By measuring the time taken for all the carbonaceous deposits on thesurface of the measurement electrode to be oxidised, it is possible todetermine the amount of carbonaceous deposit formed at the surfacethereof during the adsorption phase. Since the length of the adsorptionphase is known, it is then possible to determine the amount ofcarbonaceous deposit formed on the surface of the electrode per unittime and from this the concentration of trace organic materials in theprocess environment.

FIG. 2 illustrates a second embodiment of a sensor, in which thereference numerals refer to the same elements as indicated above, exceptthat the suffix “a” has been added to distinguish the two forms ofsensor. In this embodiment, the reference environment is provided by asolid state reference material which is sealed from the sensingenvironment by sealing material 32, typically a glass material. Thisembodiment also includes an optional counter electrode 34.

In this embodiment the current generating means 28 a passes the constantcurrent between the counter electrode 34 and the measurement electrode10 a so as to minimize errors generated in the voltage measuring device30 a. The voltage measuring device 30 a measures the voltage between themeasurement electrode 10 a and the reference electrode 16 a.

FIG. 3 illustrates the change in both the total charge passed throughthe cell and the measured potential difference across the cell duringthe passage of such charge over titration time t_(tit). The lower curverepresents total charge passed and the upper curve represents potentialdifference across the cell. It can be seen that as the total chargeincreases to a maximum value the potential difference across the celldecreases from a maximum value to a minimum constant value. The timetaken for the potential difference across the cell to change from themaximum value determined upon application of the current I_(p) to thecell to the minimum constant value is the time taken for oxidation ofall the carbonaceous deposits from the surface of the measurementelectrodes.

EXAMPLES Example 1

Construction of the Sensor

Reference, and measurement electrodes, and optional counter electrode,were formed on a thimble/disc of the oxygen anion conducting electrolyte(commercially available from various suppliers) by sputtering undervacuum or using commercially available ‘inks’ and firing the assembly ina suitable atmosphere according to the procedure given by the inkmanufacturer.

A gas tight seal (resistant to both vacuum and pressure) was formedaround the oxygen anion conducting electrolyte to isolate themeasurement electrode from the reference electrode and optional counterelectrode using standard procedures. Depending upon how the sensor is tobe heated the heater/thermocouple can be added at any appropriate stageduring manufacture.

Example 2

Determination of the TOC levels in a Process Gas

A sensor having a YSZ solid state electrolyte having a T_(c) of 300° C.and a platinum measurement electrode with a surface area (A) of 1 cm²,the electrode having a surface density, ρ, of 10¹⁵ atoms/cm², wasexposed to a process gas containing propylene at a temperature ofT_(ads) for a time t_(ads) of 10³ seconds. The average stickingprobability, S, for organic contaminants on the sensing electrode is0.1. The temperature of the sensor was raised to T_(tit) and a currentI_(p) of 2×10⁻⁶ A was passed between the reference and measurementelectrode. During the passage of the current the potential differenceacross the cell dropped to a constant value of 350 mV over a periodt_(tit) of 100 seconds. The total charge passed between the referenceand measurement electrode was thus 2.10⁻³ Coulombs, which corresponds tothe formation of 0.3 monolayers of carbonaceous deposit in accordancewith equation 11. $\begin{matrix}{M = {\frac{I_{tit}t_{tit}}{4{\mathbb{e}}}\frac{1}{A\quad\rho}}} & {{Equation}\quad 11}\end{matrix}$

The total equivalent partial pressure (mbar) of the organic contaminant(expressed as equivalents of carbon atoms) is calculated as 1.0×10⁻⁸mbar or 1.0 ppt using equations 12, 13 below. $\begin{matrix}{P_{TOC} = {\frac{M}{{St}_{ads}} \times 3.410^{- 6}}} & {{Equation}\quad 12}\end{matrix}$TOC _((ppt)) =P _(TOC)×10⁻⁹   Equation 13

1. An organic contaminant molecule sensor for use in a processenvironment having a low oxygen concentration, comprising: anelectrochemical cell comprising a solid state oxygen anion conductor inwhich oxygen anion conduction occurs at or above a critical temperature;a measurement electrode formed on a first surface of the anion conductorfor exposure to the monitored environment, the measurement electrodecomprising a material for catalyzing the dehydrogenation of an organiccontaminant molecule wherein at temperatures below the criticaltemperature, organic contaminant molecules are adsorbed onto anddehydrogenated at the surface of the material of the measurementelectrode to form a carbonaceous deposit on the surface of the materialof the measurement electrode; and a reference electrode formed on asecond surface of the anion conductor for exposure to a referenceenvironment, the reference electrode comprising a material forcatalyzing the dissociation of oxygen to oxygen anions; a heater forcontrolling the temperature of the electrochemical cell; and a currentsource for controlling the electrical current between the referenceelectrode and the measurement electrode, wherein at temperatures abovethe critical temperature, an electrical current is passed between thereference electrode and the measurement electrode to control the numberof oxygen anions passing from the reference electrode to the measurementelectrode to oxidize the carbonaceous deposits.
 2. The sensor accordingto claim 1, wherein the measurement electrode comprises a metal selectedfrom the group of metals consisting of rhenium, osmium, iridium,ruthenium, rhodium, platinum and palladium and alloys thereof.
 3. Thesensor according to claim 2, wherein the alloys include an elementsselected from the group of elements consisting of silver, gold andcopper.
 4. (canceled)
 5. The sensor according to claim 4, wherein thereference electrode comprises a metal capable of dissociating oxygen. 6.The sensor according to claim 1 wherein the solid state oxygen anionconductor is selected from the group of conductors consisting ofgadolinium doped ceria and yttria stabilized zirconia.
 7. The sensoraccording to claim 1 comprising a counter electrode positioned adjacentto the reference electrode.
 8. The sensor according to claim 7 whereinthe counter electrode comprises a metal capable of dissociating oxygen.9. The sensor according to claim 1 comprising a reference environmenthaving a gaseous source of oxygen at atmospheric pressure.
 10. Thesensor according to claim 1 wherein the reference environment comprisesa solid-state source of oxygen.
 11. The sensor according to claim 10wherein the solid state source of oxygen comprises a metal/metal oxidecompound.
 12. The sensor according to claim 1 wherein the heater furtherincludes a thermocouple assembly.
 13. The sensor according claim 1further including means for measuring a potential across the sensor. 14.The sensor according to claim 13 wherein the sensor monitors the levelsof trace organic contaminants in a low oxygen concentration monitoredprocess environment.
 15. A method of monitoring the levels of traceorganic contaminants in a process environment comprising the steps of:providing an electrochemical sensor comprising a solid state oxygenanion conductor in which oxygen anion conduction occurs at or above acritical temperature, a measurement electrode formed on a first surfaceof the conductor for exposure to the process environment, themeasurement electrode comprising a material for catalyzing thedehydrogenation of the trace organic contaminants, and a referenceelectrode formed on a second surface of the conductor for exposure to areference environment, the reference electrode comprising a material forcatalyzing the dissociation of oxygen to oxygen anions; exposing themeasurement electrode at a sensor temperature T_(ads) to the processenvironment for a time t_(ads) to cause one or more of the trace organiccontaminants to adsorb onto and dehydrogenate at the surface of themeasurement electrode thereby leading to build up of a carbonaceousdeposit at the surface of the measurement electrode; raising thetemperature of the electrochemical sensor to a value T_(tit) above thecritical temperature of the solid state oxygen anion conductor andpassing a current I_(p) between the reference electrode and themeasurement electrode for a time t_(p) sufficient for the potentialdifference across the electrochemical sensor to reach a constant valuedetermined by the equilibrium between the flux of oxygen anions arrivingat the measurement electrode and the rate of desorption of oxygen gasfrom the reference electrode; and determining, from a total charge(I_(p)t_(p)) passed through the electrochemical sensor at temperatureT_(tit), the amount of carbonaceous deposits present at the surface ofthe measurement electrode and therefore the concentration of traceorganic contaminant in the process environment.
 16. The method accordingto claim 15 further comprising the step of raising the temperature ofthe electrochemical sensor to a temperature between T_(ads) and T_(tit)prior to the step of raising the temperature of the electrochemicalsensor to T_(tit).
 17. The method according to claim 15 wherein T_(ads)is from 20 to 150° C.
 18. The method according to claim 15 whereint_(ads) is from 102 to 105 seconds.
 19. The method according to claim 18wherein t_(ads) is about 104 seconds.
 20. The method according to claim15 wherein T_(tit) from 300 to 600° C.
 21. The method according to claim15 wherein I_(p) is from 10 nA to 100 uA.
 22. The method according toclaim 15 wherein the electrochemical sensor comprises a counterelectrode positioned adjacent to the reference electrode.
 23. The methodaccording to claim 15 wherein the reference environment comprises asource of oxygen at atmospheric pressure.
 24. The method according toclaim 15 wherein the reference environment comprises a solid-statesource of oxygen.
 25. The method according to claim 24 wherein the solidstate source of oxygen comprises a metal/metal oxide compound (or ametal oxide/metal oxide) compound.
 26. The method according to claim 15comprising the step of applying a potential V_(i) across theelectrochemical sensor.
 27. The sensor according claim 12 furtherincluding a device for measuring a potential across the sensor.
 28. Thesensor according to claim 27 wherein the sensor monitors the levels oftrace organic contaminants in a low oxygen concentration monitoredprocess environment.
 29. The sensor according to claim 12 wherein themeasurement electrode comprises a metal selected from the group ofmetals consisting of rhenium, osmium, iridium, ruthenium, rhodium,platinum and palladium and alloys thereof.
 30. The sensor according toclaim 29 wherein the alloys include an element selected from the groupof elements consisting of silver, gold and copper.
 31. The sensoraccording to claim 7 wherein the heater further includes a thermocoupleassembly.
 32. The sensor according claim 31 further including means formeasuring a potential across the sensor.
 33. The sensor according toclaim 32 wherein the sensor monitors the levels of trace organiccontaminants in a low oxygen concentration monitored processenvironment.
 34. The sensor according to claim 33 wherein the referenceelectrode comprises platinum, palladium or other metal capable ofdissociating oxygen.
 35. The sensor according to claim 7 wherein themeasurement electrode comprises a metal selected from the group ofmetals consisting of rhenium, osmium, iridium, ruthenium, rhodium,platinum and palladium and alloys thereof.
 36. The sensor according toclaim 35 wherein the alloys include an element selected from the groupof elements consisting of silver, gold and copper.
 37. The sensoraccording to claim 13 wherein the measurement electrode comprises ametal selected from the group of metals consisting of rhenium, osmium,iridium, ruthenium, rhodium, platinum and palladium and alloys thereof.38. The sensor according to claim 37 wherein the alloys include anelement selected from the group of elements consisting of silver, goldand copper.
 39. The sensor according to claim 38 wherein the referenceelectrode comprises a catalyst for the dissociation of oxygen.
 40. Thesensor according to claim 39 wherein the reference electrode comprises ametal capable of dissociating oxygen.
 41. The sensor according to claim13 wherein the solid state oxygen anion conductor is selected from thegroup of conductors consisting of gadolinium doped ceria and yttriastabilized zirconia.
 42. The sensor according to claim 13 comprising areference environment having a gaseous source of oxygen at atmosphericpressure.
 43. The sensor according to claim 13 wherein the referenceenvironment comprises a solid state source of oxygen.
 44. The sensoraccording to claim 43 wherein the solid state source of oxygen comprisesa metal/metal oxide compound.
 45. The sensor according to claim 5wherein the metal capable of dissociating oxygen is selected from thegroup of metals consisting of platinum and palladium.
 46. The sensoraccording to claim 8 wherein the metal capable of dissociating oxygen isselected from the group of metals consisting of platinum and palladium.47. A sensor according to claim 10 wherein the solid state sourcecomprises a metal oxide/metal oxide compound.
 48. The sensor accordingto claim 40 wherein the metal capable of dissociating oxygen is selectedfrom the group of metals consisting of palladium and platinum.
 49. Thesensor according to claim 43 wherein the solid state source of oxygencomprises a metal oxide/metal oxide compound.